Piezoelectric element and process for production thereof

ABSTRACT

A piezoelectric element includes a ceramic substrate, a piezoelectric(s) made of a ceramic composition composed mainly of a PbMg ⅓ Nb ⅔ O 3 —PbZrO 3 —PbTiO 3  ternary system solid solution composition represented by the following general formula (1) and containing 0.05 to 10.0% by weight, based on the ceramic composition, of NiO, and electrodes electrically connected to the piezoelectric. The piezoelectric is solidly attached to the ceramic substrate directly or via part or all of the electrodes:  
     Pb x (Mg {fraction (y/3)} Nb ⅔ ) a Ti b Zr c O 3   (1)  
     wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals falling in a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000; and a process for producing such a piezoelectric element. This piezoelectric element has very high piezoelectric properties, is superior in vibration transmittability between ceramic substrate and piezoelectric, and can provide an actuator or sensor of small size and high integration.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to a piezoelectric element. Moreparticularly, the present invention relates to a piezoelectric elementwhich has very high piezoelectric properties, which is superior invibration transmittability between ceramic substrate and piezoelectric,and which can provide an actuator or sensor of small size and highintegration; as well as to a process for producing such a piezoelectricelement.

[0002] In recent years, piezoelectric elements have been used in ink jetprinter heads, speakers, microphones, etc. As the piezoelectricelements, there are known those comprising a ceramic substrate, apiezoelectric made of a ceramic composition, formed on the substrate,and electrodes electrically connected to the piezoelectric. As to theceramic composition constituting the piezoelectric, various improvedcompositions have been disclosed.

[0003] For example, a PbTiO₃—PbZrO₃ binary system solid solutioncomposition, a Pb(Mg_(⅓)Nb_(⅔))O₃—PbTiO₃—PbZrO₃ ternary system solidsolution composition and a composition wherein part of Pb and Mg ineither of the above composition is replaced by Ni, Nb, Mn, etc., weredisclosed [JP-B-50-3519, JP-B-60-102779, Journal of The American CeramicSociety; 49[11] 577 (1966)]. These compositions are improved inpiezoelectric properties(e.g. piezoelectric d constant) which are themost important factor determining the piezoelectric element properties.By using a piezoelectric made of such an improved ceramic composition, apiezoelectric element having superior piezoelectric properties isexpectable.

[0004] When a piezoelectric element is produced actually by coating apiezoelectric material made of the above ceramic composition, on aceramic substrate and then heat-treating the coated piezoelectricmaterial, however, the obtained piezoelectric has a low density;therefore, there have been pointed out such problems that thepiezoelectric element has a low flexural displacement or, when a voltageis applied thereto, it causes dielectric breakdown at the low densityarea.

[0005] Hence, there has been used a piezoelectric element produced byheat-treating beforehand a piezoelectric material made of the aboveceramic composition and then attaching this piezoelectric onto a ceramicsubstrate (JP-A-11-29357).

[0006] In this piezoelectric element, attention was paid to a fact thatuse of a ceramic substrate impairs the densification of a piezoelectricformed on the ceramic substrate, and this piezoelectric elementaccordingly improved in piezoelectric properties by means of obtaining adense piezoelectric which was heat-treated beforehand a piezoelectricmaterial made of a ceramic composition.

[0007] In this piezoelectric element, however, it is necessary to use aninorganic or organic adhesive at the time of attaching the piezoelectriconto the ceramic substrate; therefore, there have been such problemsthat the adhesive impairs the vibration transmittability between theceramic substrate and the piezoelectric or the adhesive componentsinfiltrate into the piezoelectric or the ceramic substrate,deteriorating their properties.

[0008] Further, the above-mentioned conventional ceramic compositionshave had no sufficiently satisfactory piezoelectric properties.

SUMMARY OF THE INVENTION

[0009] The present invention has been completed in view of theabove-mentioned problems and aims at providing a piezoelectric elementwhich has very high piezoelectric properties, which is superior invibration transmittability between ceramic substrate and piezoelectric,and which can provide an actuator or sensor of small size and highintegration, and a process for producing such a piezoelectric element.

[0010] The present inventor made a study in order to solve theabove-mentioned problems. As a result, the present inventor found outthat when there is used a piezoelectric material composed mainly of aPbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solid solution compositionof particular formulation and containing NiO in a particular proportion,the piezoelectric material can be densified even when it is coated on aceramic substrate and then heat-treated, whereby can be obtained apiezoelectric having very high piezoelectric properties. The presentinvention has been completed based on the above finding.

[0011] According to the present invention, there is provided apiezoelectric element comprising:

[0012] a ceramic substrate,

[0013] a piezoelectric(s) made of a ceramic composition composed mainlyof a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solid solutioncomposition represented by the following general formula (1) andcontaining 0.05 to 10.0% by weight, based on the ceramic composition, ofNiO, and

[0014] electrodes electrically connected to the piezoelectric, whereinthe piezoelectric(s) is(are) solidly attached to the ceramic substratedirectly or via part or all of the electrodes.

Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1)

[0015] wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals fallingin a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325,0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475,0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.

[0016] In the piezoelectric element of the present invention, it ispreferred that NiO is dispersed in the ceramic composition in such aconcentration gradient that the concentration of NiO becomes higher fromthe interface between the piezoelectric and the ceramic substrate or theelectrode(s) towards the thickness direction of the piezoelectric.

[0017] Pb in the ceramic composition may be replaced by at least onekind of element selected from the group consisting of Sr, Ca and Ba, by2 to 10 mole %. Pb in the ceramic composition may also be replaced by Laby 0.2 to 1.0 mole %.

[0018] The ceramic substrate preferably has a thickness of 3 μm to 1 mm,and the piezoelectric preferably has a thickness of 1 to 300 μm. Theratio of the thickness of the ceramic substrate to the thickness of thepiezoelectric (the thickness of the ceramic substrate/the thickness ofthe piezoelectric) is preferably 0.1 to 30.

[0019] According to the present invention, there is also provided aprocess for producing a piezoelectric element, which comprises:

[0020] coating, on a ceramic substrate or on an electrode(s) formed on aceramic substrate, a piezoelectric material made of a ceramiccomposition composed mainly of a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternarysystem solid solution composition represented by the following generalformula (1) and containing 0.05 to 10.0% by weight, based on the ceramiccomposition, of NiO, and

[0021] heat-treating the coated piezoelectric material in theco-presence of an atmosphere-controlling material having a formulationof the ceramic NiO content as the ceramic composition or a formulationof higher NiO content than the ceramic composition.

Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1)

[0022] wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals fallingin a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325,0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475,0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.

[0023] According to the present invention, there is also provided aprocess for producing a piezoelectric element, which comprises:

[0024] preparing a plurality of different piezoelectric materials eachmade of a ceramic composition composed mainly of aPbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solid solution compositionrepresented by the following general formula (1) and having a differentNiO content,

[0025] coating in order of the low NiO content these piezoelectricmaterials on a ceramic substrate or on an electrode(s) formed on aceramic substrate, and

[0026] heat-treating the coated piezoelectric materials.

Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1)

[0027] wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals fallingin a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325,0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475,0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.

[0028] The piezoelectric element according to the present invention canbe used in capacitors or various sensors, as a dense and smalldielectric element or pyroelectric element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIGS. 1(a)(b) schematically show an embodiment of thepiezoelectric element of the present invention.

[0030]FIG. 1(a) is a plan view and

[0031]FIG. 1(b) is a sectional view taken along the X-X′ line of FIG.1(a).

[0032] FIGS. 2(a)(b) schematically show other embodiment of thepiezoelectric element of the present invention.

[0033]FIG. 2(a) is a plan view and

[0034]FIG. 2(b) is a sectional view taken along the X-X′ line of FIG.2(a).

[0035] FIGS. 3(a)(b) schematically show still other embodiment of thepiezoelectric element of the present invention.

[0036]FIG. 3(a) is a plan view and

[0037]FIG. 3(b) is a sectional view taken along the X-X′ line of FIG.3(a).

[0038] FIGS. 4(a)(b) schematically show still other embodiment of thepiezoelectric element of the present invention.

[0039]FIG. 4(a) is a plan view and

[0040]FIG. 4(b) is a sectional view taken along the X-X′ line of FIG.4(a).

[0041] FIGS. 5(a)(b) schematically show still other embodiment of thepiezoelectric element of the present invention.

[0042]FIG. 5(a) is a plan view and

[0043]FIG. 5(b) is a sectional view taken along the X-X′ line of FIG.5(a).

[0044]FIG. 6 is a sectional view schematically showing still otherembodiment of the piezoelectric element of the present invention.

[0045]FIG. 7 is a sectional view schematically showing still otherembodiment of the piezoelectric element of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0046] The present invention is described in detail below with referringto the accompanying drawings.

[0047] 1. Piezoelectric Element

[0048] As shown in FIGS. 1(a)(b), the piezoelectric element of thepresent invention is constituted by:

[0049] a ceramic substrate 2,

[0050] a piezoelectric 1 composed mainly of a particular ceramiccomposition, and

[0051] electrodes 3 (3 a and 3 b) electrically connected to thepiezoelectric 1,

[0052] wherein the piezoelectric 1 is solidly attached to the ceramicsubstrate 2 directly or via part or all of the electrodes 3.

[0053] The present piezoelectric element is described specificallybelow.

[0054] The ceramic substrate 2 used in the present piezoelectric elementis preferably made of a material containing at least one kind selectedfrom the group consisting of stabilized zirconium oxide, aluminum oxide,magnesium oxide, mullite, aluminum nitride, silicon nitride and glass,for the heat resistance, chemical stability and insulating property.

[0055] Of these, particularly preferred is a material containingstabilized zirconium oxide, for the high mechanical strength andsuperior toughness.

[0056] The ceramic substrate 2 has a thickness of preferably 3 μm to 1mm, more preferably 5 to 500 μm, particularly preferably 7 to 200 μm.

[0057] When the thickness is less than 3 μm, the piezoelectric elementmay have a low mechanical strength; when the thickness is more than 1mm, the rigidity of the ceramic substrate 2 relative to the shrinkagestress of the piezoelectric 1 is large when a voltage is applied to thepiezoelectric element, and the piezoelectric element may show a smallflexural displacement.

[0058] The ceramic substrate 2 may be produced so as to have, as shownin FIGS. 2(a)(b), a thin portion 2 c roughly corresponding to aninterface 2 a between piezoelectric 1 or electrode 3 b and ceramicsubstrate 2 and having the above-mentioned thickness and a thick portion2 b roughly corresponding to the surface area of ceramic substrate otherthan the interface 2 a and having a larger thickness than the thinportion 2 c.

[0059] Thereby, the piezoelectric element obtained can show a largerflexural displacement and a higher mechanical strength.

[0060] It is also possible that a plurality of units each consisting ofa piezoelectric and electrodes are provided on a single large ceramicsubstrate 2, as shown in FIGS. 3(a)(b).

[0061] There is no particular restriction as to the surface shape of theceramic substrate 2. As the surface shape, there can be mentioned, forexample, a rectangle, a square, a triangle, an ellipse, a true circle,an edge-rounded square, an edge-rounded rectangle, a capsule andcombinations thereof.

[0062] The piezoelectric 1 used in the present piezoelectric element ismade of a ceramic composition composed mainly of aPbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solid solution compositionrepresented by the following general formula (1) and containing NiO in aparticular proportion.

Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1)

[0063] wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals fallingin a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325,0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475,0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.

[0064] Thereby, the ceramic composition can have improved piezoelectricproperties and the piezoelectric 1 can have a high density, making itpossible to obtain a piezoelectric element showing an improved flexuraldisplacement.

[0065] The reason why, in the general formula (1), a, b and c areallowed to fall in the above-mentioned particular range, is that, if a,b and c are outside of the range, the piezoelectric element shows asmall flexural displacement.

[0066] The NiO content in the ceramic composition is 0.05 to 10.0% byweight, preferably 0.50 to 8.0% by weight, more preferably 1.0 to 6.0%by weight.

[0067] When the NiO content is less than 0.05% by weight, thedensification of the piezoelectric is insufficient and the piezoelectricelement obtained therewith shows a small flexural displacement. When theNiO content is more than 10.0% by weight, the piezoelectric has a higherreactivity with the ceramic substrate 2 and the piezoelectric elementobtained therewith shows a small flexural displacement.

[0068] NiO is dispersed in the ceramic composition preferably uniformlyand more preferably in such a concentration gradient that theconcentration of NiO becomes higher from the interface 2 a between thepiezoelectric 1 and the ceramic substrate 2 or the electrode(s) 3towards the thickness direction of the piezoelectric 1.

[0069] Thereby, the piezoelectric 1 can have a higher density even whenit is solidly attached to the ceramic substrate 2 directly or via anelectrode(s) 3.

[0070] In order to allow the ceramic composition per se to have largepiezoelectric properties, it is preferred to replace the Pb in theceramic composition by at least one kind of element selected from thegroup consisting of Sr, Ca, Ba and La.

[0071] When the Pb in the ceramic composition is replaced by at leastone kind of element selected from the group consisting of Sr, Ca and Ba,the Pb is replaced preferably by 2 to 10 mole %, more preferably 4 to 8mole %.

[0072] When the Pb is replaced by less than 2 mole %, the resultingpiezoelectric element may show a small flexural displacement; when thePb is replaced by more than 10 mole %, the resulting piezoelectricelement has a low Curie point and its flexural displacement may changelargely with temperature.

[0073] When the Pb in the ceramic composition is replaced by La, the Pbis replaced preferably by 0.2 to 1.0 mole %, more preferably 0.4 to 0.9mole %.

[0074] When the Pb is replaced by less than 0.2 mole %, the resultingpiezoelectric element may show a small flexural displacement; when thePb is replaced by more than 1.0 mole %, the resulting piezoelectricelement has a low Curie point and its flexural displacement may changelargely with temperature.

[0075] The piezoelectric 1 preferably has an average grain diameter of 1to 20 μm, more preferably 2 to 10 μm.

[0076] When the average grain diameter is less than 1 μm, the domain inthe piezoelectric 1 does not develop sufficiently, which may result in asmall flexural displacement. When the average grain diameter is morethan 20 μm, the domain in the piezoelectric 1 is large but does not moveeasily, which may result in a small flexural displacement.

[0077] The piezoelectric 1 contains a phase(s) other than perovskitephase preferably by 20% by volume or less, more preferably by 10% byvolume or less.

[0078] When the piezoelectric 1 contains a phase(s) other thanperovskite phase by more than 20% by volume, the resulting piezoelectricelement may show a small flexural displacement.

[0079] The piezoelectric 1 has a porosity of preferably 10% by volume orless, more preferably 5% by volume or less.

[0080] When the porosity is more than 10% by volume, the resultingpiezoelectric element shows a small flexural displacement and may havesmall mechanical strengths.

[0081] The piezoelectric 1 has a thickness of preferably 1 to 300 μm,more preferably 3 to 100 μm, particularly preferably 5 to 30 μm.

[0082] When the thickness is less than 1 μm, the piezoelectric 1 made ofa particular ceramic composition used in the present invention isdifficult to densify sufficiently and the resulting piezoelectricelement may show a small flexural displacement. When the thickness ismore than 300 μm, the stress to the ceramic substrate 2 becomesrelatively too large; in order to prevent the substrate fracture, theceramic substrate 2 needs to have a large thickness, making it difficultto produce a small piezoelectric element required in practicalapplication.

[0083] The ratio of the thickness of the ceramic substrate 2 to thethickness of the piezoelectric 1 (the thickness of the ceramicsubstrate/the thickness of the piezoelectric) is preferably 0.1 to 30,more preferably 0.3 to 10, particularly preferably 0.5 to 5.

[0084] When the ratio is less than 0.1, the resulting piezoelectricelement may have small mechanical strengths. When the ratio is more than30, the resulting piezoelectric element may show a small flexuraldisplacement.

[0085] There is no particular restriction as to the electrodes 3 used inthe piezoelectric element of the present invention, except that they areelectrically connected to the piezoelectric 1. The electrodes 3 may be,for example, a pair of comb-shaped electrodes 3 c and 3 d formed on apiezoelectric 1 attached solidly to a ceramic substrate 2, as shown inFIGS. 4(a)(b); or, may be a pair of comb-shaped electrodes 3 c and 3 dattached solidly to a ceramic substrate 2 (in this case, a piezoelectric1 is solidly attached onto the electrodes 3 c and 3 d), as shown inFIGS. 5(a)(b).

[0086] Alternatively, it is possible that, as shown in FIG. 6, a commonelectrode 3 e is solidly attached onto a ceramic substrate 2, apiezoelectric 1 is solidly attached onto the common electrode 3 e, and apair of comb-shaped electrodes 3 c and 3 d are formed on thepiezoelectric 1. It is also possible that, as shown in FIG. 7, a pair ofcomb-shaped electrodes 3 c and 3 d are solidly attached onto a ceramicsubstrate 2, a piezoelectric 1 is solidly attached onto the comb-shapedelectrodes 3 c and 3 d, and a common electrode 3 e is formed on thepiezoelectric 1.

[0087] As the material for the electrodes 3, there can be mentioned atleast one kind of material selected from the group consisting ofplatinum, palladium, rhodium, silver and alloys thereof.

[0088] Of these materials, platinum or an alloy composed mainly ofplatinum is preferred because they show high heat resistance during theheat-treatment of piezoelectric 1.

[0089] The thickness of electrodes 3 are preferably 15 μm or less, morepreferably 5 μm or less.

[0090] When the thickness is more than 15 μm, the electrodes 3 act as arelaxing layer, which may result in a small flexural displacement.

[0091] The electrodes 3 are heat-treated as necessary and integratedwith the ceramic substrate 2.

[0092] In the piezoelectric element of the present invention, theabove-mentioned piezoelectric 1 is solidly attached to theabove-mentioned ceramic substrate 2 directly or via part or all of theabove-mentioned electrodes 3.

[0093] Thereby, it is possible to prevent the reduction in vibrationtransmittability between ceramic substrate 2 and piezoelectric 1, causedby the interposition of adhesive or the like, and the deterioration ofproperties of piezoelectric 1 and ceramic substrate 2, caused by theinfiltration of adhesive components or the like and the resultingreduction in flexural displacement.

[0094] Herein, “be solidly attached” refers to that the piezoelectric 1is tightly integrated with the ceramic substrate 2 directly or via partor all of the electrodes 3, owing to a solid-state reaction between theceramic substrate 2 and the piezoelectric 1 or the electrode(s) 3,without using any adhesive of organic or inorganic type.

[0095] The piezoelectric element of the present invention can beproduced by the first or second process described below.

[0096] 2. First Process

[0097] The first process of the present invention comprises coating apiezoelectric material composed mainly of a particularly ceramiccomposition, on a ceramic substrate or an electrode(s) formed on aceramic substrate, and heat-treating the coated ceramic material in theco-presence of an atmosphere-controlling material having a particularformulation.

[0098] Specific description is made below.

[0099] In the first process of the present invention, first, apiezoelectric material made of a particular ceramic composition iscoated on a ceramic substrate or an electrode(s) formed on a ceramicsubstrate.

[0100] The ceramic substrate used in the first process is the same asdescribed with respect to the piezoelectric element of the presentinvention.

[0101] The piezoelectric material used in the first process is made of aceramic composition composed mainly of a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ternary system solid solution composition represented by the followinggeneral formula (1) and containing NiO in a particular proportion.

Pb_(x)(Mg_({fraction (y/3)})Nb_(⅓))_(a)Ti_(b)Zr_(c)O₃  (1)

[0102] wherein 0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals fallingin a range surrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325,0.125), (0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475,0.425) and (0.375, 0.425, 0.200), and a+b+c=1.000.

[0103] Thereby, the ceramic composition can have improved piezoelectricproperties and the piezoelectric can have a high density, making itpossible to obtain a piezoelectric element having high piezoelectricproperties.

[0104] The reason why, in the general formula (1), a, b and c areallowed to fall in the above-mentioned particular range, is that, if a,b and c are outside of the range, the piezoelectric element shows asmall flexural displacement.

[0105] The NiO content in the ceramic composition is 0.05 to 10.0% byweight, preferably 0.50 to 8.0% by weight, more preferably 1.0 to 6.0%by weight.

[0106] When the NiO content is less than 0.05% by weight, thedensification of the piezoelectric is insufficient and the piezoelectricelement obtained therewith shows a small flexural displacement. When theNiO content is more than 10.0% by weight, the piezoelectric has a higherreactivity with the ceramic substrate and the piezoelectric elementobtained therewith shows a small flexural displacement as well.

[0107] As the raw materials for the composition represented by thegeneral formula (1), there can be mentioned, for example, simplesubstances (elements) each consisting of Pb, Ba, Ca, Sr, La, Mg, Nb, Zror Ti; oxides of these elements (e.g. PbO, Pb₃O₄, La₂O₃, MgO, Nb₂O₅,TiO₂ and ZrO₂); carbonates of these elements (e.g. BaCO₃, SrCO₃, MgCO₃and CaCO₃); and compounds containing a plurality of these elements (e.g.MgNb₂O).

[0108] These raw materials for piezoelectric material can be used singlyor in combination of two or more kinds per each elements.

[0109] There is no particular restriction as to the method for preparingthe piezoelectric material. However, the piezoelectric material can beprepared by following method, for example.

[0110] First, the above-mentioned raw materials are mixed so that thecontents of elements (Pb, Ba, Ca, Sr, La, Mg, Nb, Zr and Ti) inresulting mixture become as desired relative to the general formula (1);the mixture is mixed with NiO in the above-mentioned given proportions;the resulting mixed raw material is calcinated at 750 to 1,300° C. toobtain a desired ceramic composition.

[0111] Lastly, the calcinated piezoelectric material is ground to obtaina piezoelectric material having desired particle diameters.

[0112] The piezoelectric material may be prepared by other method, forexample, an alkoxide method or a co-precipitation method.

[0113] There is no particular restriction as to the method for coatingthe piezoelectric material. However, as the method, there can bementioned, for example, an ion beam method, sputtering, vacuumdeposition, PVD, ion plating, CVD, plating, screen printing, sprayingand dipping.

[0114] Of these, sputtering and screen printing are preferred becausethey enable simple and continuous coating in a shape and thickness ofhigh accuracy.

[0115] In the first process, a piezoelectric material can be coateddirectly on a ceramic substrate. Alternatively, an electrode(s) can beformed on a ceramic substrate and a piezoelectric material can be coatedon the electrode(s).

[0116] As the method for forming the electrode(s), there can bementioned, for example, an ion beam method, sputtering, vacuumdeposition, PVD, ion plating, CVD, plating, screen printing, sprayingand dipping.

[0117] Of these, sputtering and screen printing are preferred in view ofthe bondability between ceramic substrate or piezoelectric.

[0118] As the material used for forming the electrode(s), there can bementioned at least one kind of material selected from the groupconsisting of platinum, palladium, rhodium, silver and alloys thereof.

[0119] Of these materials, platinum or an alloy composed mainly ofplatinum is preferred because they show high heat resistance during theheat-treatment of piezoelectric.

[0120] The thickness of electrode(s) formed is(are) preferably 15 μm orless, more preferably 5 μm or less.

[0121] When the thickness is more than 15 μm, the electrode(s) act as arelaxing layer and the piezoelectric element may show a small flexuraldisplacement.

[0122] The electrode(s) is (are) heat-treated as necessary andintegrated with the ceramic substrate.

[0123] In the first process of the present invention, then, thepiezoelectric material coated on the ceramic substrate is heat-treatedin the co-presence of an atmosphere-controlling material having aformulation of the same NiO content as the ceramic composition or aformulation of higher NiO content than the ceramic composition.

[0124] Thereby, it becomes possible to prevent the vaporization of NiOpresent in the ceramic composition constituting the piezoelectricmaterial and obtain a piezoelectric wherein NiO is dispersed uniformlyor in a such a concentration gradient that the concentration of NiObecomes higher from the interface between the piezoelectric and theceramic substrate or the electrode(s) towards the thickness direction ofthe piezoelectric; and it becomes also possible to solidly attach thepiezoelectric to the ceramic substrate.

[0125] Specifically, by making the NiO content in theatmosphere-controlling material the same as the NiO content in theceramic composition constituting the piezoelectric material, it becomespossible to obtain a piezoelectric wherein NiO is dispersed uniformly;and by making the NiO content in the atmosphere-controlling materialhigher than the NiO content in the ceramic composition constituting thepiezoelectric material, it becomes possible to obtain a piezoelectricwherein NiO is dispersed in a such a concentration gradient that theconcentration of NiO becomes higher from the interface between thepiezoelectric and the ceramic substrate or the electrode(s) towards thethickness direction of the piezoelectric. Also, by controlling the NiOcontent in the atmosphere-controlling material, it becomes possible tocontrol the extent of the NiO concentration gradient in thepiezoelectric obtained.

[0126] There is no particular restriction as to the components otherthan NiO, of the atmosphere-controlling material. However, it ispreferred that the kinds and contents of other components in theatmosphere-controlling material are made the same as in the ceramiccomposition constituting the piezoelectric material, in order to preventthe vaporization of the components (other than NiO).

[0127] In the first process, the heat treatment temperature ispreferably 1,000 to 1,400° C., more preferably 1,100 to 1,350° C.

[0128] When the temperature is less than 1,000° C., the solid attachmentof the piezoelectric to the ceramic substrate may be insufficient or thedensity of the piezoelectric may be insufficient. When the temperatureexceeds 1,400° C., the vaporization amount of Pb in the piezoelectricmaterial is large and it may be difficult to obtain a piezoelectrichaving an intended composition.

[0129] The heat treatment may be conducted after formation ofelectrode(s) or before the formation.

[0130] 3. Second Process

[0131] The second process of the present invention comprises preparing apiezoelectric material made of a particular ceramic composition, coatingthe piezoelectric material on a ceramic substrate or an electrode(s)formed on a ceramic substrate by a particular method, and heat-treatingthe coated piezoelectric material.

[0132] The second process is described specifically below.

[0133] In the second process of the present invention, first, there areprepared a plurality of different piezoelectric materials each made of aceramic composition composed mainly of a particular composition andcontaining NiO in a different proportion. The ceramic composition usedin preparing the piezoelectric material can be the same ceramiccomposition as used in the first process, and the preparation of thepiezoelectric material can be conducted in the same manner as describedin the first process.

[0134] Incidentally, the second process is different from the firstprocess in that the former process uses a plurality of differentpiezoelectric materials each made of a ceramic composition of differentNiO content.

[0135] In the present second process, then, the plurality of differentpiezoelectric materials each made of a ceramic composition of differentNiO content are coated in order of the low NiO content piezoelectricmaterials on a ceramic substrate or an electrode(s) formed on a ceramicsubstrate.

[0136] Thereby, it is possible to reliably obtain a piezoelectricwherein NiO is dispersed in such a concentration gradient that theconcentration of NiO becomes higher from the interface between thepiezoelectric and the ceramic substrate or the electrode(s) towards thethickness direction of the piezoelectric.

[0137] There is no particular restriction as to the method for coatingthe piezoelectric materials, and the piezoelectric materials can becoated in the same manner as in the first process.

[0138] In the second process of the present invention, then, thepiezoelectric materials coated on the ceramic substrate or theelectrode(s) are heat-treated.

[0139] In the heat-treatment, the co-presence of anatmosphere-controlling material containing NiO is unnecessary. However,the co-presence of an atmosphere-controlling material containing NiO ispreferred because it can control the NiO concentration gradient in theresulting piezoelectric more accurately.

[0140] The heat-treatment may be conducted in one time after all of thedifferent piezoelectric materials of different NiO contents have beencoated, or may be conducted every time after each piezoelectric materialhas been coated.

[0141] The heat treatment conditions (the temperature, etc.) are thesame as in the first process.

[0142] The present invention is described specifically below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

[0143] In Examples and Comparative Examples, measurements were conductedas follows.

[0144] 1. Flexural Displacement

[0145] A voltage was applied to electrodes of a piezoelectric element soas to give an electric field of 3 kV/mm. The resulting flexuraldisplacement of the piezoelectric element was measured by a laserdisplacement tester.

[0146] 2. Porosity

[0147] The piezoelectric of each piezoelectric element obtained inExamples and Comparative Examples was examined by a scanning electronmicroscope, over a range of 50 μm of each of the surface, length andwidth; the areal proportion of pores in each of the three visual fieldswas determined; and the average of the three areal proportions wascalculated and taken as the porosity of the piezoelectric.

EXAMPLE 1

[0148] A lower electrode [1.2 mm×0.8 mm×3 μm (thickness)] composed ofplatinum was formed, by screen printing, on a Y₂O₃-stabilized ZrO₂substrate [the dimension of thin portion: 1.6 mm×1.1 mm×10 μm(thickness)]. They were subjected to a heat treatment to integrate thelower electrode with the substrate.

[0149] Thereon was coated a piezoelectric material [1.3 mm×0.9 mm×13 μm(thickness)] made of a ceramic composition containing 98.5% by weight ofPb_(1.00)(Mg_(⅓)Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and 1.5% by weight ofNiO, followed by a heat-treatment at 1,275° C. for 2 hours in theco-presence of an atmosphere-controlling material having the samecomposition as the piezoelectric material. The piezoelectric materialafter heat treatment had a thickness of 10 μm. Then, thereon was formed,by screen printing, an upper electrode [1.2 mm×0.8 mm×0.5 μm(thickness)] composed of gold. The resulting material was heat-treatedto produce a piezoelectric element.

[0150] The piezoelectric element was measured for flexural displacement,which was very large at 2.20 μm. The composition of the piezoelectricmaterial used and the flexural displacement of the piezoelectric elementobtained are shown in Table 1.

EXAMPLE 2 AND COMPARATIVE EXAMPLES 1 AND 2

[0151] Piezoelectric elements were produced in the same manner as inExample 1 except that the piezoelectric material used in Example 1 waschanged to those shown in Table 1.

[0152] The piezoelectric elements were measured for flexuraldisplacement. The piezoelectric element of Example 2 produced using apiezoelectric material having a, b and c falling in the range of thepresent invention, showed a large flexural displacement of 2.22 μm. Incontrast, the piezoelectric elements of Comparative Examples 1 and 2each using a piezoelectric material having a, b and c deviating from therange of the present invention, showed small flexural displacements of1.56 μm and 1.38 μm, respectively. The results are shown in Table 1.TABLE 1 Formulation of PbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ ternary systemsolid solution composition Pb-replacing element NiO Flexural Kind ofProportion Content displacement element (mole %) X Y a b c (wt. %) (μm)Example 1 None — 1.00 1.00 0.200 0.430 0.370 1.5 2.20 Example 2 Ca 5.01.00 1.00 0.375 0.375 0.250 5.0 2.22 Comparative None — 1.00 1.00 0.6000.300 0.100 8.0 1.56 Example 1 Comparative Sr 10.0 1.00 1.00 0.150 0.3500.500 0.5 1.38 Example 2

REFERENCE EXAMPLES 1 TO 3

[0153] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used, as piezoelectric materials, thoseobtained by mixing 97% by volume, 93% by volume or 85% by volume of apiezoelectric material (containing 98.5% by weight ofPb_(1.00)(Mg_(⅓)Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and 1.5% by weight ofNiO), with 3% by volume, 7% by volume or 15% by volume of a latex havingparticle diameters of 8 to 12 μm (this latex was oxidized and gasifiedby a heat-treatment and, thereby, generated pores at the sites where ithad existed before the heat treatment).

[0154] In the piezoelectric of Reference Example 3 having a large latexcontent in the piezoelectric and a porosity of 19%, the flexuraldisplacement was 1.77 μm; in the piezoelectric of Reference Example 2having a porosity of 10%, the flexural displacement was 2.08 μm; and inthe piezoelectric of Reference Example 1 having a porosity of 5%, theflexural displacement was 2.20 μm. Thus, as the porosity became larger,the flexural displacement was smaller. The results are shown in Table 2.TABLE 2 Latex Flexural amount Porosity Displacement (vol. %) (%) (μm)Ref. Ex. 1 3  5 2.20 Ref. Ex. 2 7 10 2.08 Ref. Ex. 3 15  19 1.77

EXAMPLE 3 AND COMPARATIVE EXAMPLES 3 AND 4

[0155] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used piezoelectric materials eachhaving a composition of NiO content shown in Table 3.

[0156] The piezoelectric elements were measured for flexuraldisplacement. In the piezoelectric element of Example 3 using apiezoelectric material of 1.5% by weight in NiO content, the porositywas small at 5% and the flexural displacement was large at 2.20 μm.

[0157] In contrast, in the piezoelectric element of Comparative Example3 using a piezoelectric material of small NiO content (0.03% by weight),the porosity was large at 13% and the flexural displacement was small at1.77 μm; and in the piezoelectric element of Comparative Example 4 usinga piezoelectric material of large NiO content (15.0% by weight), theporosity was small at 4% but the flexural displacement was small at 1.41μm. The results are shown in Table 3. TABLE 3 NiO Flexural contentPorosity Displacement (wt. %) (%) (μm) Example 3 1.5 5 2.20 Comp. Ex. 3 0.03 13  1.77 Comp. Ex. 4 15.0  4 1.41

EXAMPLE 4

[0158] A piezoelectric element was produced in the same manner as inExample 1 except that there was used, as a piezoelectric material, aceramic composition containing 98.0% by weight ofPb_(1.00)(Mg_({fraction (0.97/3)})Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and2.0% by weight of NiO.

[0159] The piezoelectric element was measured for flexural displacement,which was relatively large at 2.11 μm. Further, the dispersion state ofNiO in piezoelectric was examined by an EPMA analysis, which indicatedthat NiO was dispersed uniformly. The results are shown in Table 4.

EXAMPLE 5

[0160] A piezoelectric element was produced in the same manner as inExample 1 except that there were used a ceramic composition containing98.0% by weight ofPb₁₀₀(Mg_({fraction (0.97/3)})Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and2.0% by weight of NiO and, as a atmosphere-controlling material, aceramic composition containing 90.0% by weight ofPb₁₀₀(Mg_({fraction (0.97/3)})Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and10.0% by weight of NiO.

[0161] The piezoelectric element was measured for flexural displacement,which was larger than that of the piezoelectric element of Example 4, at2.29 μm. Further, the dispersion state of NiO in piezoelectric wasexamined by an EPMA analysis, which indicated that NiO was dispersed inthe piezoelectric in such a concentration gradient that the NiOconcentration became higher from the interface between electrode andpiezoelectric towards the thickness direction of the piezoelectric. Theresults are shown in Table 4. TABLE 4 Concentration Flexural gradient ofNiO displacement (μm) Example 4 Uniform 2.11 Example 5 Surfaceconcentration 2.29 was high.

EXAMPLE 6 AND COMPARATIVE EXAMPLES 5 AND 6

[0162] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used piezoelectric materials havingcompositions shown in Table 5.

[0163] The piezoelectric elements were measured for flexuraldisplacement. In the piezoelectric element of Example 6 wherein thecontent of Mg relative to Nb in piezoelectric material was in the rangeof the present invention (y=0.97), the flexural displacement was largeat 2.03 μm.

[0164] In contrast, in the piezoelectric element of Comparative Example5 wherein the content of Mg relative to Nb in piezoelectric material wassmall (y=0.85), the flexural displacement was small at 1.21 μm; and inthe piezoelectric element of Comparative Example 6 wherein the contentof Mg relative to Nb in piezoelectric material was large (y=1.15), theflexural displacement was also small at 1.55 μm. The results are shownin Table 5. TABLE 5 Formulation of PbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ternary system solid solution composition Pb-replacing element NiOFlexural Kind of Proportion Content displacement element (mole %) X Y ab c (wt. %) (μm) Example 6 Ba 4.0 1.00 0.97 0.370 0.380 0.250 2.0 2.03La 0.8 Comparative Ba 4.0 1.00 0.85 0.370 0.380 0.250 2.0 1.21 Example 5La 0.8 Comparative Ba 4.0 1.00 1.15 0.370 0.380 0.250 2.0 1.55 Example 6La 0.8

EXAMPLE 7 AND COMPARATIVE EXAMPLE 7

[0165] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used piezoelectric materials havingcompositions shown in Table 6.

[0166] The piezoelectric elements were measured for flexuraldisplacement. In the piezoelectric element of Example 7 wherein thecontent of Pb in piezoelectric material was in the range of the presentinvention (x=0.98), the flexural displacement was large at 2.23 μm.

[0167] In contrast, in the piezoelectric element of Comparative Example7 wherein the content of Pb in piezoelectric material was small(x=0.93), the flexural displacement was small at 1.75 μm. The resultsare shown in Table 6. TABLE 6 Formulation ofPbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ ternary system NiO Flexural solidsolution composition Content displacement Pb-replacing element X Y a b c(wt. %) (μm) Example 7 None 0.98 0.97 0.375 0.375 0.250 1.5 2.23Comparative None 0.93 0.85 0.375 0.375 0.250 1.5 1.75 Example 7

EXAMPLES 8 TO 11

[0168] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used piezoelectric materials havingcompositions shown in Table 7.

[0169] The piezoelectric elements were measured for flexuraldisplacement. In all of the piezoelectric elements of Examples 8 to 11,the flexural displacements were relatively large at 1.89 to 2.16 μm.

[0170] In the piezoelectric element of Example 9 using an piezoelectricmaterial made of a composition wherein 5.0 mole % of Pb was replaced bySr and the piezoelectric element of Example 10 made of a compositionwherein 10.0 mole % of Pb was replaced by Ba, the flexural displacementswere 2.16 μm and 2.10 μm, respectively, and were larger than that (2.02μm) of the piezoelectric element of Example 8 using a piezoelectricmaterial made of a composition wherein no replacement of Pb was made.

[0171] In contrast, in the piezoelectric element of Example 11 using anpiezoelectric material made of a composition wherein 7.5 mole % of Pbwas replaced by Sr and 7.5 mole % of Pb was replaced by Ca (total 15mole % of Pb was replaced), the flexural displacement was 1.89 μm andsmaller than that (2.02 μm) of the piezoelectric element of Example 8using a piezoelectric material made of a composition wherein noreplacement of Pb was made. The results are shown in Table 7. TABLE 7Formulation of PbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ ternary system solidsolution composition Pb-replacing element NiO Flexural Kind ofProportion Content displacement element (mole %) X Y a b c (wt. %) (μm)Example 8 None — 1.00 0.97 0.375 0.375 0.250 1.0 2.02 Example 9 Sr 5.01.00 0.97 0.375 0.375 0.250 1.0 2.16 Example 10 Ba 10.0 1.00 0.97 0.3750.375 0.250 1.0 2.10 Example 11 Ba 7.5 1.00 0.97 0.375 0.375 0.250 1.01.89 Ca 7.5

EXAMPLES 12 TO 14

[0172] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used piezoelectric materials havingcompositions shown in Table 8.

[0173] The piezoelectric elements were measured for flexuraldisplacement. In all of the piezoelectric elements of Examples 12 to 14,the flexural displacements were relatively large at 1.78 to 2.20 μm.

[0174] In the piezoelectric element of Example 13 using an piezoelectricmaterial made of a composition wherein 0.8 mole % of Pb was replaced byLa, the flexural displacement was 2.20 μm and was larger than that (2.02μm) of the piezoelectric element of Example 12 using a piezoelectricmaterial made of a composition wherein no replacement of Pb was made.

[0175] In contrast, in the piezoelectric element of Example 14 using anpiezoelectric material made of a composition wherein 1.5 mole % of Pbwas replaced by La, the flexural displacement was 1.78 μm and wassmaller than that (2.02 μm) of the piezoelectric element of Example 12.The results are shown in Table 8. TABLE 8 Formulation ofPbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ ternary system solid solutioncomposition Pb-replacing element NiO Flexural Kind of Proportion Contentdisplacement element (mole %) X Y a b c (wt. %) (μm) Example 12 None —1.00 0.97 0.375 0.375 0.250 1.0 2.02 Example 13 La 0.8 1.00 0.97 0.3750.375 0.250 1.0 2.20 Example 14 La 1.5 1.00 0.97 0.375 0.375 0.250 1.01.78

EXAMPLES 15 AND 16 AND COMPARATIVE EXAMPLE 8

[0176] Piezoelectric elements were produced in the same manner as inExample 1 except that there were used a piezoelectric materialcontaining 98.5% by weight ofPb₁₀₀(Mg_(⅓)Nb_(⅔))_(0.20)Ti_(0.43)Zr_(0.37)O₃ and 1.5% by weight of NiOand atmosphere-controlling materials having formulations shown in Table9.

[0177] The piezoelectric elements were measured for flexuraldisplacement. In the piezoelectric element of Example 15 produced in theco-presence of an atmosphere-controlling material having the sameformulation as the ceramic composition constituting the piezoelectric,the flexural displacement was large at 2.20 μm; and in the piezoelectricelement of Example 16 produced in the co-presence of anatmosphere-controlling material having a formulation of higher NiOcontent than that of the ceramic composition constituting thepiezoelectric, the flexural displacement was even larger at 2.42 μm.

[0178] In contrast, in the piezoelectric element of Comparative Example8 produced in the co-presence of an atmosphere-controlling materialhaving a formulation of lower NiO content than that of the ceramiccomposition constituting the piezoelectric, the flexural displacementwas 2.03 μm and was smaller than those of the piezoelectric elements ofExamples 15 and 16. The results are shown in Table 9. TABLE 9Formulation of PbMg_(1/3)Nb_(2/3)O₃—PbZrO₃—PbTiO₃ ternary system solidNiO Flexural solution composition content displacement Pb-replacingelement X Y a b c (wt. %) (μm) Example 15 None 1.0 1.0 0.200 0.430 0.3701.5 2.20 Example 16 None 1.0 1.0 0.200 0.430 0.370 8.0 2.42 ComparativeNone 1.0 1.0 0.200 0.430 0.370 0.03 2.03 Example 8

[0179] As described above, the present invention can provide apiezoelectric element which has very high piezoelectric properties,which is superior in vibration transmittability between ceramicsubstrate and piezoelectric, and which can provide an actuator, sensor,etc., all of small size and high integration; and a process forproducing such a piezoelectric element.

What is claimed is:
 1. A piezoelectric element comprising: a ceramicsubstrate, a piezoelectric(s) made of a ceramic composition composedmainly of a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solid solutioncomposition represented by the following general formula (1) andcontaining 0.05 to 10.0% by weight, based on the ceramic composition, ofNiO, and electrodes electrically connected to the piezoelectric, whereinthe piezoelectric(s) is(are) solidly attached to the ceramic substratedirectly or via part or all of the electrodes.Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1) wherein0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals falling in a rangesurrounded by (a,b,c) (0.550, 0.425, 0.025), (0.550, 0.325, 0.125),(0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and(0.375, 0.425, 0.200), and a+b+c=1.000.
 2. A piezoelectric elementaccording to claim 1, wherein NiO is dispersed in the ceramiccomposition in such a concentration gradient that the concentration ofNiO becomes higher from the interface between the piezoelectric(s) andthe ceramic substrate or the electorde(s) towards the thicknessdirection of the piezoelectric(s).
 3. A piezoelectric element accordingto claim 1, wherein Pb in the ceramic composition is replaced by atleast one kind of element selected from the group consisting of Sr, Caand Ba, by 2 to 10 mole %.
 4. A piezoelectric element according to claim1, wherein Pb in the ceramic composition is replaced by La by 0.2 to 1.0mole %.
 5. A piezoelectric element according to claim 1, wherein thepiezoelectric(s) has(have) a thickness of 1 to 300 μm.
 6. Apiezoelectric element according to claim 1, wherein the ceramicsubstrate has a thickness of 3 μm to 1 mm.
 7. A piezoelectric elementaccording to claim 1, wherein the ratio of the thickness of the ceramicsubstrate to the thickness of the piezoelectric (the thickness of theceramic substrate/the thickness of the piezoelectric) is 0.1 to
 30. 8. Aprocess for producing a piezoelectric element, which comprises: coating,on a ceramic substrate or on an electrode(s) formed on a ceramicsubstrate, a piezoelectric material made of a ceramic compositioncomposed mainly of a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solidsolution composition represented by the following general formula (1)and containing 0.05 to 10.0% by weight, based on the ceramiccomposition, of NiO, and heat-treating the coated piezoelectric materialin the co-presence of an atmosphere-controlling material having aformulation of the same NiO content as the ceramic composition or aformulation of higher Ni content than the ceramic composition.Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1) wherein0.95≦x≦1.05: 0.8≦y≦1.0; a, b and c are decimals falling in a rangesurrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125),(0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and(0.375, 0.425, 0.200), and a+b+c=1.000.
 9. A process for producing apiezoelectric element, which comprises: preparing a plurality ofdifferent piezoelectric materials each made of a ceramic compositioncomposed mainly of a PbMg_(⅓)Nb_(⅔)O₃—PbZrO₃—PbTiO₃ ternary system solidsolution composition represented by the following general formula (1)and having a different NiO content, coating these piezoelectricmaterials in order of the low NiO content piezoelectric materials on aceramic substrate or on an electrode(s) formed on a ceramic substrate,and heat-treating the coated piezoelectric materials.Pb_(x)(Mg_({fraction (y/3)})Nb_(⅔))_(a)Ti_(b)Zr_(c)O₃  (1) wherein0.95≦x≦1.05; 0.8≦y≦1.0; a, b and c are decimals falling in a rangesurrounded by (a,b,c)=(0.550, 0.425, 0.025), (0.550, 0.325, 0.125),(0.375, 0.325, 0.300), (0.100, 0.425, 0.475), (0.100, 0.475, 0.425) and(0.375, 0.425, 0.200), and a+b+c=1.000.